(a) Field of the Invention
The present invention relates to a selective catalyst reduction system that is disposed in an exhaust system of a diesel vehicle, and more particularly to a monitoring system for a selective catalyst reduction system that satisfies the reinforced OBD regulations of North America and Europe.
(b) Description of the Related Art
OBD (on board diagnostics) regulations demand improvements in diagnostic ability for detecting malfunctions and degradation of components as well as exhaust gas quality. Further, the OBD demands for satisfying a variety of provisions that are related to the standardization in the A/S (after service) market.
Exhaust gas that is exhausted out of an engine passes through a catalyst reduction system that is disposed in the middle of an exhaust pipe and is purified therein, and the noise thereof is reduced while passing through a muffler and is then expelled to the atmosphere.
The selective catalyst reduction system (SCR) is applied so as to satisfy the reinforced exhaust gas regulations of a diesel vehicle.
The selective catalyst reduction system uses ammonia NH3 as a reducing agent so as to purify NOx that is exhausted out of the diesel vehicle in a large quantity. The ammonia has a good selectiveness to the NOx, and even though oxygen exists, the ammonia reacts with well the NOx.
A chemical reaction of the NOx with ammonia NH3 is as follows, wherein the NH3 resolves the NOx into N2 and H2O.NOx+NH3(ammonia)→N2+H2O  [Reaction Formula 1]
A urea solution is injected upstream of the selective catalyst reduction system and the ammonia is created through vaporization.
A NOx sensor is provided in the selective catalyst reduction system, and the reduction efficiency of the NOx is changed based on the NOx amount that is measured through the NOx sensor through feedback control in which injection amount of the urea solution is determined.
The NOx sensor not only enhances the reduction efficiency of the NOx, but is also used to monitor the selective catalyst reduction system.
The injection amount of the urea solution may be determined and the SCR system may be monitored based on a NOx map instead of the NOx sensor. However, the precision of the NOx map decreases in a quick acceleration or quick deceleration condition, so it is difficult to satisfy the exhaust gas regulations.
The NOx reduction efficiency (η) of the selective catalyst reduction system is defined as in the following Equation 1.η=1−y/x  (Equation 1)
In Equation 1, x is the NOx amount that is measured upstream of the selective catalyst reduction system, and y is the NOx amount that is measured downstream of the same.
Also, the NOx amount that is not purified in the selective catalyst reduction system, that is, the slip rate (θ) of the NOx, is defined as in the following Equation 2.θ=1−η  (Equation 2)
Also, a deviation error of the NOx sensor disposed in the selective catalyst reduction system includes a slope offset that has a value of ±α as shown in (a) of FIG. 5 and a drift offset that has a value of ±β as shown in (b) FIG. 5, and these are defined as in the following Equation 3 and Equation 4.x=(1±α)×xo±β  (Equation 3)y=(1±α)×yo±β  (Equation 4)
In Equation 3 and Equation 4, xo represents the NOx amount that is detected upstream of the selective catalyst reduction system, yo represents the NOx amount that is detected downstream of the selective catalyst reduction system, “α” represents the slope offset of the NOx sensor, and “β” represents the drift offset of the NOx sensor.
Also, when the NOx sensors are disposed upstream and downstream of the selective catalyst reduction system, the NOx amount that is measured through the NOx sensor that is disposed in the rear is defined as in the following Equation 5 through Equation 2 to Equation 4.y=(1±α)×θ{(1±α)×xo±β}±β  (Equation 5)
For example, ynor is the NOx amount that is measured downstream of the normal selective catalyst reduction system, ythr is the threshold value of the NOx amount, and a difference Δy between the two values is defined as in the following Equation 6.Δy=ythr−ynor>0  (Equation 6)
In the worst condition in which the difference between the above two values is minimized, ynor has to have a maximum value and ythr has to have a minimum value, and these conditions are defined as in the following Equation 7 and Equation 8.ynor=(1+α)×θnor{(1+α)×xo+β}+β  (Equation 7)ythr−(1−α)×θthr{(1−α)×xo−β}−β  (Equation 8)
Equation 7 and Equation 8 are calculated into the following Equation 9.Δy2sensors={(1−α)2θthr−(1+α)2θnor}xo−{(1−α)θthr+(1+α)θnor}β−2β>0  (Equation 9)
Generally, the exhausted NOx amount is regulated to under 0.05 g/mile based on the regulations of North America (Tier2 Bin5), and it demands that the raw emission of the engine is to be under 0.3 g/mile so as to satisfy the regulation.
Accordingly, the reduction efficiency of the selective catalyst reduction system has to be at least 83.33%.
Also, in 2013, the regulation value will change to under 0.0875 g/mile, which is 1.75 times the threshold of the OBD II, such that the reduction efficiency of the selective catalyst reduction system has to become at least 70.83%.
If it is assumed that there is no slip of the ammonia NH3 in the selective catalyst reduction system, the EURO 6 regulation value, the OBD regulation value, the reduction efficiency (η) of NOx, the slip rate (θ) of NOx and so on are as in the following Table 1.
TABLE 1North AmericanOBDregulation valueEURO6Remarksregulation(g/mile)(g/km)NOx regulationItemvalue0.050.08value2010-2012 2.5 times0.1250——2013 -1.75 times0.0875——2014 -1.75 times—0.140 —Engine output—0.30 0.186 Engines are sameEfficiency ofηnor—0.83330.5709—normal SCRNOx slip rate ofθnor—0.16670.4291—normal SCRThresholdηnor2.5 2.5 times0.5833——efficiencyThreshold NOx slipθnor2.50.4167——rate of normal SCRThresholdθnor1.751.75 times0.70830.2490—efficiency ofnormal SCRThreshold of NOxθnor1.750.29170.7510—slip rate of SCR
The NOx amount of raw emission in the North America engine is assumed to be 100 ppm so as to analyze the result.
In the North America engine, the measured NOx amount is 50 ppm on average upstream of the selective catalyst reduction system, and the measured NOx amount is 12 ppm downstream of the selective catalyst reduction system.
2.5 times the threshold of the North America OBD II is to be satisfied until 2012 so as to monitor the selective catalyst reduction system in a case in which two NOx sensors are disposed.
Accordingly, Δy2sensors in Equation 9 is shown in FIG. 6.
When the NOx amount in the inlet of the selective catalyst reduction system is 100 ppm as a criterion value, the permissible drift offset (β) of the NOx sensor can be 5% in a case in which the slope offset (α) of the NOx sensor is 10%, and the permissible drift offset (η) of the NOx sensor can be 1% in a case in which the slope offset (α) is 20%.
Accordingly, when the precision of the NOx sensor is very high, it is possible to monitor the selective catalyst reduction system in accordance with the OBD II regulations.
The NOx sensor that is manufactured by Siemens does not have a slope offset, and only has ±10% drift offset in a fresh condition (not used) and ±20% drift offset in an aged condition (used).
Accordingly, the permissible limit of the drift offset is 9.7% in a case in which the slope offset is zero, so a new NOx sensor that has a drift sensor error of ±10% can properly monitor the selective catalyst reduction system, but an old NOx sensor that has a drift sensor error of ±20% cannot properly monitor the selective catalyst reduction system.
Also, in 2013, the regulation value will change to 1.75 times the threshold of the North America OBD II, and the Δy2sensors value in Equation 9 is shown as in FIG. 7.
In FIG. 7, when the NOx amount of the inlet side of the selective catalyst reduction system is 100 ppm and the slope offset does not exist, the permissible limit of the drift offset of the NOx sensor is only 5.1%.
Accordingly, in this case there is a problem that the new NOx sensor cannot monitor the selective catalyst reduction system.
Also, as 1.75 times the threshold of the OBD of the EURO 6 is to be satisfied as of 2014, the Δy2sensors in the Equation 9 is shown as in FIG. 8.
The NOx regulation of the EURO 6 is looser than the North America Tier2 Bin5, so the permissible limit of the drift offset of the NOx sensor is 10.2 ppm as shown in FIG. 8.
However, the present NOx sensor cannot monitor the selective catalyst reduction system to the level of the regulation of North America.
The above explanation is arranged as in the following Table 2, and the present NOx sensor cannot monitor the selective catalyst reduction system as shown in Table 2 against the OBD regulations of North America and Europe.
TABLE 2ItemNorth AmericaEuropeRemarksOBD II2.5X1.75X1.75XOBD regulationregulation(from 2012)(from 2013)(from 2014)NOximpossibleimpossibleimpossibleThe new NOxsensorsensor is possible,but impossiblein a realcondition
That is, when the degradation of the NOx sensor is considered, the present NOx sensor cannot satisfy the OBD regulation of North America and Europe.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.